Saturday, July 24, 2010

Anodic Stripping Voltammetry (ASV)

Electrochemical assays for arsenic detection such as this is very promising. This method is suitable for detection in liquid samples such as groundwater. It can be applied to solid samples as well, but the sample has to be digested or extracted before testing.

ASV is also suitable for measuring dissolved arsenic in drinking water and it is equally sensitive towards As (III) and As (V).

Although ASV can be used to monitor other kinds of elements, we will now only discuss on how it detects arsenic. Basically, it works based on the principle of electroplating arsenic onto an electrode, which concentrates it. The arsenic that is electroplated or reduced onto the electrode is then stripped off or oxidized off. We can control this electroplating and stripping off action by raising or lowering the potential, which will be discussed in detailed in the procedures. The stripping off action generates a current that can be measured. The current (milliamps) is proportional to the amount of arsenic being stripped off.

As ASV can be used to monitor other types of metals besides arsenic, it is necessary for us to identify the metal that is being stripped off; the potential (voltage in millivolts) will allow us to determine the characteristic for each metal. This allows us to both identify and quantify the metal that is being measured.

Anodic stripping voltammetry usually incorporates three electrodes, a working electrode, auxiliary electrode (sometimes called the counter electrode), and reference electrode. An electrolyte is usually essential for most samples. For most standard tests, the working electrode is a mercury film electrode. The mercury film forms an amalgam(mixture) with the analyte(the substance or sample being analyzed) of interest, which upon oxidation results in a sharp peak, improving resolution between analytes. The mercury film is formed over a glassy carbon electrode. A mercury drop electrode has also been used for much the same reasons. In cases where the analyte of interest has an oxidizing potential above that of mercury, or where a mercury electrode would be otherwise unsuitable, as the analyte will not be stripped off easily as it cannot be easily oxidized. Hence, we can solve this problem by using a solid, inert metal such as silver, gold, or platinum may also be used.

The detailed procedures are shown as below:

1. The solution is continuously stirred during the first 2 steps. The first step is the cleaning step where the potential is raised to a higher potential for a period of time to fully strip the metal off from the electrode.

2. The potential is then lowered to a lower potential so as to reduce the metal and deposit it on the electrode. After this second step, the stirring is then stopped.

3. If a mercury electrode is used, more time should be allocated to make sure that the deposited material is distributed evenly onto the electrode. If a solid inert electrode is used, this step may be skipped.

4. Lastly, the working electrode is then raised to a higher potential and the metal is stripped off or oxidized. This stripping action will give off electrons, which is a measure of the current.